It is known that, if one considers a rotor blade so that its successive elementary sections along the wingspan have a constant chord and that the chords of said elementary sections are coplanar, the elementary lift and drag forces connected to each of said elementary blade sections vary appreciably, just like the square of the distance of a given elementary section to the axis of rotation of the rotor. As a result, the extremity zone of the blade has a significant effect on the aerodynamic functioning of the rotor and the resulting aerodynamics of the lift and drag forces occur on about 75% of the wingspan of the blade.
Moreover, it is known that, so as to better adapt the incidence of successive elementary sections to air speeds the latter encounter owing to rotation of the blade, the twist of such a blade around its longitudinal axis is generally carried out so as to make the latter operate at high pitch close to the rotation axis of the rotor where the speed is low and at low pitch where the speed is higher. Such a twist is generally linear, in other words the elementary variation d .theta. of the twist angle .theta. on a wingspan variation dr is constant. By means of such a measurement, it is possible to improve the working fineness Cz/Cx of profiles along the wingspan of the blade. However, as proved from many experiments, an extremely intense edge vortex is formed at the blade extremity resulting from the compensation of pressure differences existing between the lower wing surface and the upper wing surface of sections close to the extremity. The presence of this edge vortex leads firstly to an increase of the power consumed by the rotor and secondly to a significant emission of noise, both at low running speeds by interaction between the vortex and the following blades which intercept it, and high running speeds owing to extreme running aerodynamic conditions accompanied by the appeararance of air compressibility in these conditions and the presence of shock waves.